People walk past a gold store on Nov. 27, 2017, in Istanbul, Turkey. The trial of Reza Zarrab, an Iranian-Turk who ran a foreign exchange and gold dealership, got underway in New York on Tuesday. (Chris McGrath/Getty Images)

Turkish gold dealer Reza Zarrab pleaded guilty Tuesday to unspecified charges against him in a deal that requires him to testify at the U.S. trial of his co-defendant, Mehmet Hakan Atilla.

Zarrab and Atilla, the deputy CEO of Turkey’s state bank, were arrested in the U.S. last year and charged with trading cash from Zarrab’s companies and Turkish state-run banks for gold to buy oil from Iran, which has been prohibited by sanctions.

The case has resulted in strained relations between the U.S. and Turkey, with Turkish officials accusing the U.S. of holding Zarrab “hostage,” the New York Daily News reported.

Zarrab had been expected to cut a deal since summer, with rumors flying that he is cooperating with special counsel Robert Mueller’s investigation of former National Security Adviser Michael Flynn for alleged secret lobbying work for the Turkish government.

Turkish President Recep Tayyip Erdogan has claimed the prosecution of the case is an attempt to undermine the Turkish economy.

Zarrab is expected to testify against Atilla late Tuesday or Wednesday, Bloomberg reported.

In Turkish markets, the Lira fell 1.7 percent and Halkbank shares fell 7.9 percent while yields on 10-year bonds in Turkey were spiking to over 13 percent.

Zarrab had reportedly stopped cooperating with the defense in the trial weeks ago, fueling speculation about a deal or cooperation with Mueller, Bloomberg noted. Seven others have been charged in the conspiracy, but only Zarrab and Atilla are in U.S. custody.

“Atilla is just another one of Reza Zarrab’s many victims,” said Atilla’s lawyer, Victor Rocco, adding that Atilla became caught up in a “swirling, huge international storm of intrigue, lies and massive corruption,” Bloomberg reported.